Clinical and experimental data suggest that the progressive course of chronic renal disease may be altered favorably by restriction of the dietary intake of both protein and phosphorus. Therefore, the National Institutes of Health is sponsoring the Cooperative Clinical Study of Dietary Modification on the Course of Progressive Renal disease (MDRD) to test this hypothesis. The specific aims of the study are three-fold: 1. To compare and investigate the efficacy of a low-protein diet and phosphorus restriction, a very low protein diet with dietary supplements and phosphate restriction, and a diet with moderate protein intake and phosphorus restriction on the course of progressive chronic renal insufficiency. 2. To determine patient acceptance of these dietary interventions. 3. To evaluate the nutritional safety and nutritional and other effects of these various dietary interventions. To be included in the study patients must be 18-75 years of age with their GFR in the general range of 10-80 ml/min/1.73 m2. Exclusion criteria include pregnancy, history of poor medical compliance, abnormal body weight, hypoalbuminemia, proteinuria greater than 10 grams/24 hours, uncontrolled hypertension, insulin requiring diabetes mellitus, malignancy, severe heart failure, chronic liver and lung disease, systemic infections, active collagen vascular disease, urinary retention and allergy to iothalamate or iodine. Patients selected for the study will undergo extensive diet and nutritional evaluation, and measurement of renal function. After a three-month baseline period patients are randomized to a specific dietary regimen and followed monthly for a minimum period of 15 months. Nutrition and renal function will be monitored carefully. Several "stop" based on evaluation of nutritional status, renal function and general medical condition, are built into the protocol to insure patient safety.